An SOI structure generally comprises at least one intermediate insulating layer between a thin upper layer and a carrier substrate, typically made of silicon. The expression “thin layer” is understood to mean a layer that is typically between 50 Å and a few microns in thickness, for example, about 120 Å in thickness.
The insulating layer may be an oxide layer, such as a layer of SiO2, which is then called a BOX for buried oxide, i.e., the oxide is buried under a thin silicon layer. The carrier substrate is sometimes made of another material, for example, of sapphire—SOS (silicon-on-sapphire) structures are then spoken of.
Indeed, certain heterostructures do not comprise an intermediate insulating layer, as is the case for DSB (direct silicon bonding) substrates in which a thin silicon layer having a first crystal orientation is joined to a second silicon substrate having a second crystal orientation that is different to the first.
The fabrication of SOI structures generally comprises the following steps:                forming an insulating layer of a first substrate and/or a second substrate;        bonding, preferably molecular bonding (a.k.a. direct bonding), the first substrate to the second substrate; and        removing a back part of one of the two substrates so as to leave only a thin layer, also called the useful layer, on the insulating layer.        
Fabrication processes furthermore comprise bond-strengthening and finishing steps for improving the surface finish of the thin layer thus obtained. The finishing steps may be, for example, polishing or annealing steps.
In the microelectronics field, the quality of the thin layer, and that of the bonding interface, are important. In particular, it is desirable to reduce the number of defects as far as possible in order to allow electronic components to be produced on or in these structures.
However, current processes for fabricating heterostructures may cause a number of different types of defects. Among these defects, blisters and voids are particularly problematic.
These defects are especially generated by the outgassing of species, most commonly hydrogen (H2) or even helium (He) from layers of the structure, and accumulation of the outgassed species, in particular, at the bonding interface.
This outgassing may result from steps of implanting species such as hydrogen or helium ions, for example, in the context of implementation of the SMARTCUT® process.
It may also be generated during the bonding step or during steps of strengthening the bond between the two substrates. Specifically, during the strengthening anneal, water molecules react with the materials of the first and second substrate (possibly by diffusing through an optional superficial oxide layer) via an oxidation reaction, which may be written, in the case of silicon substrates, as:2H2O+Si→SiO2+2H2.
This reaction, therefore, produces molecules of hydrogen gas that are trapped in the buried oxide layer, when the latter is present, which thus acts as a reservoir of hydrogen gas.
However, in the case of an ultrathin oxide layer, or when this layer is absent altogether, it is not possible for all the hydrogen gas molecules generated to be stored, and the excess accumulates at the bonding interface and generates defects.
Specifically, as soon as the temperature to which the bonded structure is subjected to exceeds 300° C., the hydrogen gas begins to exert pressure on defects present at the bonding interface, causing blisters to form.
This effect is described in the following articles: “A model of interface defect formation in silicon wafer bonding,” S. Vincent et al., Applied Physics Letters 94, 101914 (2009), and “Study of the formation, evolution, and dissolution of interfacial defects in silicon wafer bonding,” S. Vincent et al., Journal of Applied Physics 107, 093513 (2010).
It would, therefore, be advantageous to be able to limit, as far as possible, the number of defects resulting from this outgassing effect.
The smaller the thickness of the BOX, to the point where there is no BOX, such as is the case for DSB heterostructures, for example, the more problematic this effect becomes. In particular, the latest generation of SOI structures, called UTBOX (ultra-thin buried oxide) structures, in which the insulating layer is smaller than about 50 nm in thickness, exhibits a high defect density because the insulating oxide layer is not thick enough to contain all the gas freed during the process.
To solve this problem, it has been suggested, in U.S. Pat. No. 7,485,551, to implant, in the oxide layer of the SOI structure, atoms that can trap the gaseous species liable to generate defects in the structure. This solution, nevertheless, has the drawback that the implanted atoms disrupt the SOI structure.
It is also known from U.S. Pat. No. 7,387,947, that attempts have been made to use an amorphous silicon layer to trap the gas.
Finally, from JP 2007318097, the use is known of a polysilicon layer, placed near the insulating layer so as to trap metal species liable to contaminate the oxide layer. This technique does not directly solve the problem of how to trap species generated during the fabrication process. Furthermore, with this technique, the polysilicon may recrystallize with a large grain size during the heat treatment, thus affecting the uniformity and the functionality of the substrate.